The present invention relates to a process for die-casting light-weight metals, in particular aluminum and aluminum alloys, in which the molten metal is charged into a filling chamber and injected from the filling chamber into a hollow mold cavity by means of a piston.
In a known die-casting process molten metal is introduced into a filling chamber and, by means of a piston, injected from the filing chamber into a hollow mold cavity of a die-casting machine. The greater part of the gases such as e.g. air or water vapor is expelled from the mold cavity by the metal injected into the mold. In variants of this process, the mold cavity is evacuated in advance down to a residual pressure of approx. 200 to 500 mbar and, in special vacuum die-casting processes, even to a residual pressure of less than 100 mbar.
Molds for die-casting thin walled and large surface area or complex shaped die-cast parts exhibit narrow regions which hinder the melt and make it practically impossible to remove the gases from the mold cavity. On evacuating the mold before filling it is not possible to achieve a high vacuum, because of the lack of air tightness and due to the cost and time involved. Although the occlusion of gases in the form of pores or blisters is much less pronounced with vacuum die-casting than with conventional die-casting, the number of these defects in the die-cast part is still too high for the use of such parts as safety components in automobile manufacture, because of inadequate mechanical properties.
In a die-casting process for casting aluminum parts known by the name Pore Free Die-casting (PFD), before injecting the metal into the mold cavity, the latter is flooded with oxygen, to a pressure above atmospheric pressure so that the gases in the mold cavity are replaced by oxygen. The oxygen fed to the mold cavity flows through narrow gaps and regions and, after a certain duration of flooding, the greater part of the gases previously in the mold cavity are expelled from the mold cavity and it is possible to prevent atmospheric gases from re-entering the mold. On subsequently injecting of molten aluminum into the mold, the aluminum reacts with oxygen to form Al.sub.2 O.sub.3 which remains as a dispersion of fine particles in the die-cast part without noticeably altering its properties.
It has been found, however, that even on maintaining a pressure in the mold cavity above the atmospheric pressure, it is practically impossible to completely remove the gases from the interior of a die-casting mold by flooding it with oxygen. Residual gases often remain for an extended period in regions that are difficult to flood. Water-based separating agents require, for example, a certain amount of time until they dry up completely under relatively high atmospheric pressure. In the case of die-casting molds for manufacturing die-cast pacts of relatively complicated shape, some regions are difficult to reach with oxygen with the result that residual gases such as air or water vapor are not replaced by oxygen, but remain as such in the mold cavity. During die-casting, these residual gases and water vapor form separating agents remaining in the mold cavity and become trapped in the metal form pores there and, as a result of subsequent heat treatment such as e.g. solution treatment lead to blisters at the surface. Due to these blisters, many die-casting parts cannot be heat treated.
Accordingly, it is a principle object of the present invention to provide a process for die-casting as described above wherein the occlusion of gases is reduced considerably and as a result the above mentioned problems of formation of pores and blisters in die-cast parts can be prevented.